The Actual Benefits of Intermittent Fasting, According to Experts - GQ

Every couple of years, like the very clockwork it depends on, intermittent fasting resurfaces as a dietary trend. "It’s really interesting that it has held this kind of unusual fascination as a trend for so many years, as it isn't anything new from a clinical nutrition perspective,” says Stacie Stephenson, DC, CNS, board member of the American Nutrition Association. To be fair, though, intermittent fasting is one of the more beneficial and universally useful nutrition strategies out there—just not for the reason most people think. “Do I consider intermittent fasting a weight loss diet? No, I don't,” Dr. Stephenson says. “But I think it's a really great tool.” A tool for what, then, you might ask? Here’s everything you need to know. What is intermittent fasting? “Intermittent fasting is as simple as not eating for half the day,” says Dr. Stephenson. “For 12–14 hours of your day, you’re not consuming anything other than water. That is as simple as it is.” Basically, with intermittent fasting, half or more of your day is spent in a fasted state, with a specific window designated for eating. When that window occurs, and how long it lasts, is up to you—nutritionists recommend up to 12 hours, but no fewer than eight—as long as you’re consistent. "What became known as intermittent fasting was really the concept of a window in which you consume calories,” says Ashley Koff, RD, nutrition course director for UC Irvine’s Susan Samueli Integrative Health Institute’s Integrative and Functional Medicine Fellowship. “It came about as a way to double down on explaining to people that you should not be consuming calories all the time.” How can intermittent fasting benefit your body? First and foremost, intermittent fasting is intended not for weight loss, not for building muscle, but as a way to restore some law and order to your body’s core processes. “The reason that fasting has gotten so much attention is that, in the industrialized era, and certainly with access to food becoming available 24 hours a day, we’ve really blurred the boundaries of when we’re consuming calories,” says Koff. “We’re not really meant to be consuming calories when our body is in its recovery mode, doing its cleanup work and resting. And so the concept was to give yourself guardrails: This is the time when I start eating, and this is the time when I stop eating. And that's what became intermittent fasting.” "It's helpful for one simple reason, which probably sounds really boring, which is it rests your digestive system,” says Dr. Stephenson. But that’s a lot more important than it sounds. As simple as it may seem, taking control of when you eat—and, more importantly, when you don’t—is an upstream intervention with implications that cascade all throughout your body. It’s like flicking a single domino that sets a dozen Rube Goldberg machines into motion. “Resting your digestive system reduces total body inflammation, which leads to better health and provides you with longevity,” Dr. Stephenson says. “And when I say longevity, I don't mean living to 120. I mean more healthful living—being fit, not being ill, having energy, balanced hormones, balanced insulin, and stable blood glucose.” Can intermittent fasting help you lose weight? Anytime a nutrition trend blows up on social media, people assume it’s supposed to help you lose weight. So it’s worth emphasizing, for those in the back, that intermittent fasting is not a weight-loss diet. "I’d say the biggest reason that people turn to intermittent fasting is for weight loss,” says Dr. Stephenson. “It sounds like an easy fix. ‘Oh, I just don't eat for 12 hours, and then I eat my normal diet—that's led to the weight gain—in the other 12.’ That's really faulty reasoning.” Ultimately, weight loss comes down to calories in and calories out. The timing of when you eat those calories has little to no bearing on that equation. In fact, a 2020 study published in the journal JAMA found that overweight adults who kept all their meals to within an eight-hour window for 12 weeks didn’t lose significantly more weight than the control group, leading the study authors to conclude that “time-restricted eating, in the absence of other interventions, is not more effective in weight loss than eating throughout the day.” However, while intermittent fasting is not technically a weight-loss strategy, it may have some indirect benefits. This mostly comes down to its ability to help regulate blood sugar. “When you start to control and reintegrate that system, so that you've got a healthier and more sustained relationship with insulin, you start to reignite insulin sensitivity, and then the hormones around that,” says Colin Robertson, PhD, nutritionist, exercise physiologist, and chief product officer at Zinzino. The key thing to remember, if weight loss is indeed your priority, is that intermittent fasting alone won’t move the needle by much unless you also address your actual diet. “I've had a lot of people come in and be like, ‘I'm intermittent fasting, and I don't know why I haven't lost weight,’” says Koff. “You could do intermittent fasting and still drink too much, overeat, and not get enough fiber. If the quantity that you're consuming, and the quality of what you're consuming, isn't also better for you, just addressing the timing is only one piece of the puzzle.” What can intermittent fasting can do for your mind and energy levels? A couple of years ago, I decided to give intermittent fasting a try for a couple of weeks. Ultimately, it was my undeniable love of brunch that led to my undoing—but not before I noticed an unexpected side effect. I felt sharper, mentally. And when I brought this up with friends and colleagues who’d also experimented with intermittent fasting, it turned out I wasn’t alone. “When you're not digesting all the time, there’s actually more energy available and left over for your brain,” says Dr. Stephenson. “It's like how after a meal you're not usually at your brightest. You eat food, and glucose goes up. So insulin goes up to drive the glucose down. But in the time it takes to equalize back to a steady state of blood glucose, which can be kind of long—30 minutes to as much as two hours after a meal—that's where people report that kind of brain fog feeling.” According to Koff, another way that intermittent fasting might improve cognition—particularly if you’re cutting yourself off a few hours before bed—is by optimizing sleep efficiency, which has been shown to improve brain function across the board, from decision making to memory consolidation. "The majority of humans stand to benefit from having very clear guardrails around their recovery window,” Koff says. "Let's say you’re getting about eight hours of sleep. We really don't want to be giving the body extra work—as in, digestive activity—while we're sleeping. So let's add two to three hours of fasting before bed, so your body isn't digesting when you're trying to sleep.” Who should try intermittent fasting? While almost anyone could benefit from intermittent fasting, it’s not recommended for everyone. That includes athletes, or anyone serious about performance. If you’re training for Hyrox, getting ready for your first half-marathon, or on a quest to boost your bench press, food is more than sustenance—it’s fuel. “Whether you're in training or actually competing, I think it's really important to be more situational with your nutrition,” Koff says. That means bookending workouts with proper pre- and post-workout nutrition, and whatever else your body needs to perform at its best and recover quickly. Similarly, Koff recommends skipping intermittent fasting any time you’re in a state of healing, whether that be coming back from an injury, recovering from surgery, or undergoing treatment. “For anything like that,” she says, “you're much better off working with your practitioner on a personalized plan that's going to give your body what it needs.”